The present invention concerns a process for multi-stage generation of vacuum and condensation and recycling of the vapors withdrawn from the polycondensation of precondensates in the production of polyesters. The vapors contain at least one C.sub.2 to C.sub.4 alkyl glycol and reaction by-products of the polycondensation process. The last stage of the vacuum generation process compressing to atmospheric pressure, consists of a liquid ring pump, a cooler, and a degasifier tank. Each of the at least two preceding stages includes a glycol vapor jet and a downstream glycol spray condenser.
Polyesters are produced by reacting at least one dicarboxylic acid such as terephthalic acid or naphthalene dicarboxylic acid or the methyl esters thereof with at least one alkyl glycol such as ethylene glycol or 1,4-butanediol, to which optionally small amounts of a cyclical or aromatic diol such as 1,4-cyclohexanedimethanol may be added. The compounds are reacted by esterification or ester exchange and then precondensed in a moderate vacuum, splitting off water and a small amount of glycol. Finally polycondensation is performed at a high vacuum, splitting off glycol and a small amount of water.